The well-known TESI (Transfer of Electrostatic Image) system of photoreproduction utilizes the creation of a latent, electrostatic image on a dielectric surface. This image comprises zones of electric charge selectively laid down on the surface of the dielectric sheet. After production of the electrostatic image, the electrostatic image surface is brought into contact with a toner material such as carbon black in combination with an adhesive resin, under such conditions that the toner material adheres to the electrostatically-charged portions of the dielectric surface, and does not adhere to the remaining portions. Accordingly, a visible image is formed on the dielectric surface.
A large amount of research has been invested into the TESI process, as well as other processes involving the creation of an electrostatic latent image on a dielectric surface. Examples of patents relating to this area of technology include: Carlson, et al. U.S. Pat. No. 2,982,647 Schaffert U.S. Pat. No. 3,147,679, Robinson U.S. Pat. No. 3,598,579, and the prior art of record in those patents.
In the TESI process, light is shined through a negative onto a photoconductive electrode. The light-transmitting portions of the negative permit activation of the light-irradiated portions of the photoconductive electrode, causing its electrical resistance to decrease. As a result of this, electrical current can selectively flow through the irradiated portions of the electrode across a small air gap of micronic size, to charge portions of the dielectric surface directly adjacent to the conducting portions of the photoconductive electrode.
In the co-pending U.S. patent application Ser. No. 502,628 of Oleg Szymber, entitled "Means for Improving The Contrast Of An Electrostatic Latent Image", filed simultaneously herewith, it is proposed to apply an electrical potential between the photosensitive electrode and the dielectric surface as a plurality of discrete pulses of similar polarity, separated by time periods in which the applied electrical potential is reduced to essentially zero. In this method, the irradiation of the photoconductive electrode may continue during the time periods of zero applied potential, but the irradiation is only of selected portions of the photoconductive electrode.
In accordance with this invention, significant improvements in the contrast and quality of photocopies made by many latent electrostatic image processes can be achieved by exposing the grounded photoconductive electrode to irradiation over its entire surface in the periods of zero applied electrical potential which follow the discrete pulses of electrical potential.
The effect of this latter irradiation, which is preferably applied as a brief pulse from a strobe lamp or the like while the photoconductive electrode is grounded, is not only to improve the contrast, apparently by removing residual electrical charge from the areas of the photoconductor corresponding to the non-irradiated portions of the photoconductive electrode, but most unexpectedly, it can also create effects which can be used to modify the latent electrostatic image so that the intensity of the resulting visual image created by toning of the electrostatic image may be significantly increased.
This method, with its unexpected results, appears to be directly contrary to previous examples of the use of light to erase latent, electrostatic images, such as in Smith, et al. U.S. Pat. No. 3,776,632; Fujitsuka, et al. U.S. Pat. No. 3,778,148; or Ohta, et al. U.S. Pat. No. 3,697,172.